1. Field of the Invention
The invention relates to a method for the simultaneous production of acid and base of high purity through the electrodialytic splitting of a corresponding salt in aqueous solution using an electrodialysis cell. The invention also relates to an electrodialysis cell for carrying out the method.
2. Description of the Related Art
In a number of chemical process steps, salt solutions accumulate and, as such, are not directly used further or cannot or should not be introduced into a drainage canal as waste either. Furthermore, salt solutions with high concentrations are obtained in leaching processes of salt deposits or in the leaching of salts which are already conveyed as well as pure prepared salts. In many cases, it is in the interest of chemical engineering and economy to produce from such salt solutions more highly refined valuable substances in the form of acids and bases corresponding to the ions of the respective salt. Electrolytic or electrodialytic methods are frequently used for that purpose. The known electrodialytic methods used for that purpose operate with a three-chamber system (see report of Fraunhofergesellschaft: Institut fxc3xcr Grenzflxc3xa4chen- und Bioverfahrenstechnik [Fraunhofer Association: Institute for Interface and Bio-Material Processing], April 1999, xe2x80x9cElektrodialyse mit bipolaren Membranenxe2x80x9d [Electrodialysis with Bipolar Membranes]).
In that respect, the salt solution which is to be prepared electrodialytically is conducted through a middle chamber of an electrodialysis cell being formed of three chambers. The cations travel from that cell, under the influence of the electrical field, through a cationic exchanger membrane into an adjacent chamber which contains the cathode, and form the base there with cathodically developed OHxe2x88x92-ions. Accordingly, the anions travel through an anionic exchanger membrane into the adjacent anode chamber on the other side and form the corresponding acid with the H+-ions developed anodically there. However, the production methods of acids and bases from salt solutions, which operate according to that method, have disadvantages. One disadvantage resides in the fact that unwanted reactions with anions take place at the anode, which lead to the contamination of the acid being formed. In that way, for example, hydrohalic acids, formed in the anode chamber with free halogens which are produced at the anode by the discharging of halide ions, are contaminated and their service value is therefore reduced. Moreover, the anode can be corrosively attacked or the ion exchanger membranes can be damaged by the halogen being released. Another disadvantage resides in the fact that the anodically formed acids are frequently not sufficiently concentrated and are therefore of little value in terms of chemical engineering and commerce.
In U.S. Pat. No. 4,212,712 a method is described for the electrodialytic production of a more highly concentrated sodium hydroxide solution from sodium chloride solutions in a three-chamber cell. However, with that method, hydrochloric acid is not directly electrodialytically formed, but instead, chlorine is separated anodically. The intermediate chamber lying between the anode chamber and the cathode chamber is separated from both adjacent chambers by cationic exchanger membranes which, in addition to being permeable to the Na+-ions, are permeable to water to differing degrees. The permeability to water is less towards the cathode region than the anode region into the intermediate chamber. Through the use of that configuration it is possible to generate a comparatively concentrated sodium hydroxide solution in the cell. In the authoritative literature it is also mentioned that it would be possible to use, in place of one intermediate chamber, two or more such intermediate chambers which are equipped in the direction of the cathode chamber with cationic exchanger membranes, that are permeable to water to an increasingly poorer extent. In that way, the sodium hydroxide solution in the cathode chamber is concentrated even more. However, in practice such a solution is not used because of the associated difficulties in achieving a satisfactory efficiency of flow.
It is accordingly an object of the invention to provide a method and an electrodialysis cell for the simultaneous production of acid and base through the electrodialytic splitting of a corresponding salt in an aqueous solution using an electrodialysis cell, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and with which unwanted anode effects can be avoided and acids and alkalis can be produced with comparatively high concentrations and high purity. In particular, it is an object of the method according to the invention to produce, from sodium chloride solutions, hydrochloric acid of high purity and in concentrations which were heretofore not accessible with electrodialytic measures on an industrial scale.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the simultaneous production of acid and base of high purity by the electrodialytic splitting of a corresponding salt in aqueous solution with an electrodialysis cell, which comprises providing a cathode chamber having a cathode, an inlet opening and at least one outlet opening for fluids. A salt chamber is separated from the cathode chamber by a cationic exchanger membrane. The salt chamber has an inlet opening and an outlet opening for conducting a salt solution. An acid is formed in an acid chamber separated from the salt chamber by an anionic exchanger membrane. The acid chamber does not contain an anode. An anode chamber is separated from the acid chamber by a cationic exchanger membrane through which protons required for forming the acid pass from the anode chamber into the acid chamber. The anode chamber has an inlet opening and an outlet opening for a liquid proton carrier flowing through the anode chamber. The anode chamber has a hydrogen-consuming anode for converting hydrogen into protons to an extent required for forming the acid. An electrical voltage is applied between the anode and the cathode for maintaining an electrodialytic process. Cations of a salt travel under the effect of the electrical field, from the salt chamber, through the cationic exchanger membrane into the cathode chamber and form a base there with OHxe2x88x92-ions produced by catholytic splitting of water into hydrogen and OHxe2x88x92ions. Simultaneously, anions of the salt travel from the salt chamber, under the effect of the electrical field, through the anionic exchanger membrane into the acid chamber and form the acid there with protons formed analytically from hydrogen at the hydrogen-consuming anode.
With the objects of the invention in view, there is also provided an electrodialysis cell for the simultaneous production of acid and base of high purity from a corresponding salt by electrodialysis, comprising a cathode chamber having a cathode, an inlet opening and at least one outlet opening for fluids. A salt chamber is separated from the cathode chamber by a cationic exchanger membrane. The salt chamber has an inlet opening and an outlet opening for conducting a salt solution. An acid chamber in which an acid is formed is separated from the salt chamber by an anionic exchanger membrane and does not contain an anode. An anode chamber is separated from the acid chamber by a cationic exchanger membrane through which protons required for forming the acid pass from the anode chamber into the acid chamber. The anode chamber has an inlet opening and an outlet opening for a liquid proton carrier flowing through the anode chamber and a hydrogen-consuming anode for converting hydrogen into protons to an extent required for forming the acid. A device applies an electrical voltage between the anode and the cathode for maintaining an electrodialytic process. The device simultaneously causes cations of a salt to travel under the effect of the electrical field, from the salt chamber, through the cationic exchanger membrane into the cathode chamber and form a base there with OHxe2x88x92-ions produced by catholytic splitting of water into hydrogen and OHxe2x88x92ions, and causes anions of the salt to travel from the salt chamber, under the effect of the electrical field, through the anionic exchanger membrane into the acid chamber and form the acid there with protons formed from hydrogen at the hydrogen-consuming anode.
The chamber in which the acid is formed is separated from the anode region by a cationic exchanger membrane due to the introduction of a fourth chamber into the known three-chamber cell. This avoids a situation where component parts which are located in the chamber in which the acid is formed and which could enter into electrochemical reactions at the anode arrive at the anode and are converted there to form disturbing impurities which remain in the acid. An example of such an impurity is elemental, dissolved chlorine which can be formed by the discharging of chloride ions at the anode. very generally, with the suggested configuration very pure acids can be produced because only the substances which can pass through the cationic exchanger membrane out of the anode chamber and which can pass through the anionic exchanger membrane out of the middle, salt-carrying chamber can arrive by way of the membranes, acting in an ion-selective manner, in the region in which the acid is formed. Moreover, as tests have shown, the service life of the anode is increased several times with the new configuration.
It is also surprisingly possible for the first time, with the method and device according to the invention, to use an electrodialytic device to produce hydrochloric acid on an industrial scale, having a concentration which amounts to over 10% by weight HCl in the acid. Efforts by the inventors to obtain pure hydrochloric acid with concentrations over 10% by weight HCl with conventional three-chamber systems had failed. With the new cell structure it is now possible to produce hydrochloric acid with concentrations to over 20% by weight HCl in the hydrochloric acid and at the same time sodium hydroxide solution with concentrations over 30% by weight NaOH in the alkali. The method can also be carried out with two or more electrodialysis cells connected in parallel. The method operates particularly effectively if two or more electrodialysis cells are connected in series.
In order to carry out the method, the solution of a salt, for example the aqueous solution of NaCl, NaBr, KCl, KBr, KNO3, NaNO3 or an acetate which is preferably somewhat acidified, is conducted into the chamber of the electrodialysis cell which is limited on one side by a cationic exchanger membrane and on the other side by an anionic exchanger membrane. The chamber which contains the cathode and in which the base or alkali is formed, is connected to the cationic exchanger membrane. The chamber in which the acid is formed is connected to the anionic exchanger membrane. However, this chamber does not contain any anode. The anode is located in a further, fourth chamber which is connected to the chamber in which the acid is formed. Both latter chambers are separated by a cationic exchanger membrane. When the salt solution passes through the chamber into which it has been introduced, cations of the salt dissolved in it constantly pass through the cationic exchanger membrane into the cathode chamber of the cell. The cathode can be formed of any material which is resistant to alkalis and which is suitable for the reaction:
2 H2O+2 exe2x88x92xe2x80x94H2+2 OHxe2x88x92.
Such electrodes are commercially obtainable and are formed, for example, of nickel or of titanium and are activated with precious metal oxides from the group Pt, Pd, Rh, Ru, Ir, Re, Au or with titanium oxides. Advantageously, the cathodes which are used have as low an overvoltage as possible for the aforementioned reaction for the formation of hydrogen. A commercially available nickel cathode activated without the addition of foreign elements or foreign oxides has proven to be particularly advantageous. A diluted aqueous solution of the base is in the cathode chamber. A portion of the water of the solution is split at the cathode into hydroxyl ions and hydrogen to the extent that cations enter into the cathode chamber. The cations form the base together with the hydroxyl ions. The diluted solution is concentrated by this process. The alkali is continuously drawn off from the cathode chamber. The hydrogen which is likewise produced is taken and is supplied from the cathode chamber by a pipeline to the anode, which is constructed as a hydrogen-consuming anode, for conversion into positively charged hydrogen ions, that are referred to below as protons. Negative charges in the form of anions of the salt pass through the anionic exchanger membrane into the chamber in which the acid is formed, to the extent that positive charges in the form of cations pass into the cathode chamber. A diluted proton acid flows through the chamber in which the acid is formed. This proton acid is characterized by the anion which is also the anion of the salt that is to be electrodialytically split. The protons which are required for forming the acid enter from the anode chamber through the cationic exchanger membrane into the chamber in which the acid is formed, to the extent that anions enter into this chamber. The diluted acid which is initially present in the chamber is concentrated by this acid formation process. To the extent that protons travel into the chamber in which the acid is produced, the corresponding number of protons is formed from hydrogen at the hydrogen-consuming anode.
Any purchasable anode which is suitable for generating protons from water could be used as the anode. However, depolarized hydrogen-consuming electrodes are used, with the aid of which protons can be generated from gaseous hydrogen. A lower cell voltage can be used with the use of this anode type, because only the standard hydrogen potential of 0 volt is required for the conversion of hydrogen into protons. However, with water-decomposing anodes the higher potential of +1.21 V is required for the reaction 2 H2Oxe2x80x944 H++O2. Suitable gas diffusion anodes can be purchased (for example from the firm E-TEK, Inc., Natick, Mass.). A diluted proton acid which can be dissociated well, preferably an acid from the group sulfuric acid, perchloric acid, phosphoric acid, is disposed in the anode chamber as a transport medium for the protons. This acid is not consumed. Nevertheless, it is advantageous to circulate it by pumping. Membranes made from a polymer or copolymer, which is doped with an anionic ligand, for example of the sulphonic acid group, and which has been produced from monomers from the group tetrafluoroethylene, hexafluoropropylene, monochlorotrifluoroethylene, vinylidene fluoride and xcex1,xcex2,xcex2-trifluorostyrene, are preferably used as the cationic exchanger membranes. Such membranes are commercially obtainable under the trademark NAFION (owned by Du Pont). The use of NAFION 117 for the cationic exchanger membrane between the anode chamber and the chamber in which the acid is formed, and of NAFION 324 for the cationic exchanger membrane which separates the salt-carrying chamber from the cathode chamber, has proven advantageous for the present invention. However, the invention is not restricted to the use of the named types of cationic exchanger membranes. Other types of cationic exchanger membranes, for example NAFION 115, Fumatec FKF, FKC, PKL and FKE or Tokuyama Alkali CMS, CIMS, CM-2 or Asahi Class SELEMION CAV, CSV can also be used. Commercially available membranes are likewise used as anionic exchanger membranes between the salt-carrying chamber and the chamber in which the acid is formed. The membrane ACM of Tokoyama Alkali has proven advantageous. However, other membranes such as, for example, the membranes AHA, AMH and ACS of Tokoyama Alkali, or the qualities FAB and FAA of the firm Fumatec or the membranes PCAPC Acid 35 and PC Acid 35 PEEK of the firm PCA GmbH or the membrane AAV of Asahi Glass SELEMION or the membrane ARA of Morgan, can also be used. The cell voltage when carrying out the method also depends on the cell construction, in addition to the standard potentials and overvoltage effects to be observed. It lies in the range of 1.5 to 6 V. It is advantageous to set the process temperature to 40xc2x0 C. or more, preferably to 80xc2x0 C. It is, of course, also possible to produce, according to the method of the invention, acids and alkalis from salts such as, for example, Na2SO4, NaHSO4, or phosphates, which have already been electrodialyzed according to the known three-chamber method.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for the simultaneous production of acid and base of high purity, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.